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'''Causes'''
The reason that haemophilia mostly affects males is that it is [[sex-linked]]. Women have two X-chromosomes whereas men have one X and one Y. It does not matter if there is a defect in a woman's X-chromosome, as the equivalent [[allele]] in her other chromosome would express itself and she would not have the illness. However the Y-chromosome in men has no [[gene]] for factors VIII or IX. If the [[recessive]] gene responsible for blood clotting in a man's X-chromosome is deficient there is no equivalent on the Y-chromosome, so the deficient gene is not masked by the [[dominant gene|dominant]] allele and he will develop the illness.
This means also that although women can be carriers of the illness, if men have the gene they also have the illness. It is possible, though rare, for a woman to have the illness, if her father is a haemophiliac and her mother a carrier, or if there is a defect in one of her X chromosomes and she inherits a haemophilia gene from one of her parents. This situation is far more common today than it once was, as improved treatment for haemophilia means that more men survive to adulthood and become parents. Adult women with haemophilia [[menstruation|menstruate]] periodically, so they must take clotting factor to survive.
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